Water-decomposable absorbent article and method for producing it

ABSTRACT

Provided is a water-decomposable absorbent article including an absorbent body that has a water-decomposable top sheet, a water-decomposable back sheet, a water-decomposable adsorbent core sandwiched between the top sheet and the back sheet, and an adhesive layer provided on the back sheet to fit the absorbent article to an external support; and a water-decomposable release sheet for protecting the adhesive layer before use with covering the adhesive layer. The release sheet includes a release layer that faces the adhesive layer, a water-decomposable fibrous layer, and a water-soluble polymer layer sandwiched between the release layer and the fibrous layer. A basis weight of the release layer is at least 0.05 g/m 2  but smaller than 0.3 g/m 2 , and an equilibrium contact angle between a surface of the release layer and a water drop thereon is at least 100 degrees.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention The present invention relates to awater-decomposable absorbent article for pantiliners, sanitary napkins,incontinence pads, diapers, etc.

[0002] 2. Description of the Related Art

[0003] Recently, water-decomposable absorbent articles disposable inflush toilets have come available, including, for example, pantiliners,sanitary napkins, incontinence pads, diapers, etc. Such awater-decomposable absorbent article comprises a water-decomposable topsheet to face the skin of a wearer (i.e., the top sheet serves as a bodyfacing surface), a water-decomposable back sheet to face an externalsupport such as an underwear (i.e., the back sheet serves as a garmentfacing surface), and a water-decomposable absorbent core sandwichedbetween the top sheet and the back sheet. In general, the back sheet isprovided with an adhesive layer via which the absorbent article isfitted to the external support such as an underwear, and the adhesivelayer is covered with a release sheet so as not to degrade itsadhesiveness until use. Before use, the release sheet is peeled off toexpose the adhesive layer outside, and the absorbent article is fittedto the external support via the thus-exposed adhesive layer.

[0004] Water-decomposable absorbent articles have self-sustainingstrength enough to keep their shape during use, and, after used, theyare disposed of in toilets and readily decomposed in water. However, therelease sheet which is to be peeled off before use, is made of a resinsheet or a cardboard sheet coated thick with silicone, and is verytroublesome since it must be treated as wastes. If the release sheet isalso made from a water-decomposable material, it can be disposed of intoilets without requiring treating as wastes, and thereby beingconvenient to users.

[0005] In that situation, Japanese Unexamined Patent Publication (Kokai)No. Heisei 11-47187 discloses a water-decomposable individual packagingabsorbent article, for which is used a water-decomposable release sheet.The water-decomposable release sheet used for it is made of awater-soluble resin film or a water-soluble paper coated with a siliconeor Teflon for release treatment, and the absorbent article isindividually wrapped with the release sheet.

[0006] However, the water-soluble resin film coated with a silicone orTeflon for release treatment is problematic in that, when it is storedfor a long period of time in high-humidity environments, the filmabsorbs water and swells, and, as a result, the strength of the wrappingbag of the film is lowered. Another problem with it is that the swollenfilm will cause surface defects such as pin holes through the releasetreatment portion of silicone or the like. Such surface defects tend tozip the release sheet when the release sheet is released from theadhesive layer of the absorbent article, and, as a result, smoothlyreleasing the release sheet from the adhesive layer will be oftendifficult. In case where the water-soluble resin film is made thick toincrease its wet strength, such a thick film will gel in water whendisposed of in flush toilets, and will clog drainpipes. Thewater-soluble paper coated with the lubricant (silicone) is alsoproblematic in that the hydrophobic, three-dimensionally crosslinkingrelease layer of silicone or Teflon will bond fibers of thewater-soluble paper to each other, and, as a result, the release sheetwill be poorly or hardly decomposable in water.

[0007] Japanese Unexamined Patent Publication (Kokai) No. Heisei 8-48950discloses a release sheet prepared by sizing a water-decomposable paperwith a water-soluble sizing agent followed by coating a surface thereofwith silicone. However, the invention disclosed in this patentpublication is essentially directed to a technique of recycling usedpaper for regenerated paper, and not to a technique of ensuring rapiddecomposition in water of silicone-coated water-decomposable paper andof preventing the surface defects of the coating film of silicone.Japanese Unexamined Patent Publication (Kokai) No. Heisei 6-218006discloses a water-degradable release liner prepared by coatingwater-degradable paper with a solution of polyvinylpyrrolidone, dryingit, and further coating it with a lubricant silicone layer. In thedisclosed release liner, however, the polymer material ofpolyvinylpyrrolidone is not heat-sealable. Therefore, when the releaseliner is tried to individually wrap absorbent articles with it, itcannot be formed into bags through heat-sealing viapolyvinylpyrrolidone.

[0008] In those disclosed in the above-mentioned Japanese UnexaminedPatent Publication (Kokai) Nos. Heisei 6-218006 and 8-48950, awater-soluble resin is infiltrated into the fiber-to-fiber space inwater-decomposable paper, or water-decomposable paper is coated with awater-soluble resin solution thereby to size the paper. However, thecomposite film of water-decomposable paper and a water-soluble resin,which is produced by infiltrating the water-soluble resin intowater-decomposable paper or by coating water-decomposable paper with thewater-soluble resin solution in the manner disclosed, is stillproblematic in that a surface of the composite film tends to beroughened owing to the roughness of the constituent fibers. In casewhere a release layer of silicone or the like is formed on such a roughsurface of the composite film, it could not have a uniform thicknesssince the underlying composite film does not have a flat surface. As aresult, when the water-soluble resin in the composite film has absorbedwater to swell, the release layer that overlies the composite film willhave surface defects such as pin holes. Still another problem with thecomposite film produced by coating water-decomposable paper with thewater-soluble resin solution is that the composite film is often warpedin the step of drying it after coated with the resin solution, owing tothe difference in the wet expansion coefficient between thewater-decomposable paper and the water-soluble resin. As a result, whena release layer of crosslinked silicone or the like is formed on such awarped composite film, it will be cracked or will have surface defectssuch as pin holes.

[0009] Through the surface defects (cracks and pin holes) of the releaselayer, water will penetrate into the underlying water-soluble resinlayer to swell or gel it, and the thus-swollen or gelled resin layerwill be exposed outside through the surface defects of the releaselayer. If so, the release properties of the release layer forfacilitating easy release of the release sheet from an adhesive layerwill be thereby lowered, and, as a result, smoothly peeling the releasesheet from the adhesive layer of an absorbent article will be difficult.

SUMMARY OF THE INVENTION

[0010] An object of the invention is to provide a water-decomposableabsorbent article provided with an adhesive layer to which is fitted awater-decomposable release sheet, wherein the release sheet is improvedso that its release layer is flat and is resistant to water, that it iswell decomposable in water and that its releasability from the adhesivelayer is surely kept for a long period of time.

[0011] Another object of the invention is to provide such awater-decomposable absorbent article, in which the release sheet can beformed into a bag for packaging the absorbent article and it can beheat-sealed via a water-soluble polymer layer that underlies the releaselayer.

[0012] According to one aspect of the invention, a water-decomposableabsorbent article may comprise: an absorbent body that includes awater-decomposable top sheet, a water-decomposable back sheet, awater-decomposable adsorbent core sandwiched between the top sheet andthe back sheet, and an adhesive layer provided on the back sheet to fitthe absorbent article to an external support; and a water-decomposablerelease sheet for protecting the adhesive layer before use with coveringthe adhesive layer, wherein;

[0013] the release sheet includes a release layer that faces theadhesive layer, a water-decomposable fibrous layer, and a water-solublepolymer layer sandwiched between the release layer and the fibrouslayer,

[0014] a basis weight of the release layer is-at least 0.05 g/m²butsmaller than 0.3 g/m², and an equilibrium contact angle between asurface of the release layer and a water drop thereon is at least 100degrees.

[0015] Preferably, the release layer is made from a silicone compoundlayer, and its weight is at most 0.15 g/m².

[0016] The release sheet for the water-decomposable absorbent article ofthe invention comprises a release layer of silicone or the like formedon a water-decomposable fibrous layer of, for example,water-decomposable paper or water-decomposable non-woven fabric, via awater-soluble polymer layer therebetween. In this, therefore, thelubricant constituting the release layer does not penetrate into thefiber-to-fiber space in the fibrous layer to bond the constituentfibers, and the decomposability in water of the release sheet is notlowered. When the release sheet is disposed of in flush toilets, waterpenetrates into the water-soluble polymer layer existing between thefibrous layer and the release layer, to thereby dissolve the polymerlayer, and, after all, the release layer is readily peeled off from thethus-dissolved polymer layer in water jets.

[0017] In the invention, the weight of the release layer is specificallydefined to fall within the range as above, and, in addition, theequilibrium contact angle between the surface of the release layer and awater drop thereon is specifically defined to be at least 100 degrees.Being so defined, the release layer is flat and is resistant to water.Therefore, even when the release sheet is stored under a high-humiditycondition for a long period of time or water adheres thereto in theprocess of producing it, or while its release layer is kept in contactwith a water-soluble adhesive, water hardly penetrates into itswater-soluble polymer layer via its release layer. As a result, thewater-soluble polymer that constitutes the polymer layer is preventedfrom being swollen or gelled by water, as little water penetrates intothe layer, and the release layer is hardly damaged to have surfacedefects such as pin hole. For these reasons, the release sheet ensuresgood releasability from the adhesive layer for a long period of time. Inaddition, the release layer is readily dispersed in water, as theuppermost limit of its weight is defined as set forth above.

[0018] Preferably, the thickness of the water-soluble polymer layerfalls between 5 μm and 20 μm. Also preferably, the water-soluble polymerlayer is formed from a polyvinyl alcohol, and the polyvinyl alcohol forthe layer has a degree of saponification of at least 70%.

[0019] In case where such a water-soluble polymer layer of which thethickness falls within the defined range is formed on thewater-decomposable fibrous layer, the influence of the surface roughnessof the fibrous layer on the polymer layer can be reduced thanconventionally, and the release layer can be well formed on the polymerlayer in a more stable condition. In addition, the polymer layer ofwhich the thickness falls within the defined range can reduce the stressto warp the release sheet.

[0020] Also preferably, the release sheet is to form a bag to entirelywrap and envelop the absorbent article therein, and its outer peripheryis heat-sealed via the polyvinyl alcohol layer to form the bag.

[0021] According to another aspect of the invention, the method forproducing the water-decomposable absorbent article that includes awater-decomposable top sheet, a water-decomposable back sheet, awater-decomposable absorbent core sandwiched between the top sheet andthe back sheet, and an adhesive layer provided on the back sheet to fitthe absorbent body to an external support, may comprise;

[0022] a step of melt-extruding and laminating a water-soluble polymerlayer having a thickness of from 5 μm to 20 μm on a surface of awater-decomposable fibrous sheet made of hydrophilic and water-insolublefibers,

[0023] a step of applying a release material onto a surface of thewater-soluble polymer layer to form thereon a release layer having aweight of at least 0.05 g/m² but smaller than 0.3 g/m²,

[0024] a step of crosslinking the release material such that anequilibrium contact angle between the surface of the release layer and awater drop thereon is at least 100, degrees, thereby forming a releasesheet, and

[0025] a step of fitting the release sheet to the adhesive layer of theabsorbent body via the release layer.

[0026] In the method, the water-soluble polymer layer is melt-extrudedand laminated on the fibrous layer. In this, therefore, it is easy todefine the thickness of the polymer layer to fall within the desiredrange.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a perspective view of one embodiment of an absorbentarticle of the invention;

[0028]FIG. 2 is a cross-sectional view of the absorbent article of FIG.1, cut along the line II-II;

[0029]FIG. 3 is a perspective view of the absorbent article of FIG. 1,wrapped and enveloped in a bag of a release sheet;

[0030]FIG. 4 is a cross-sectional view of another embodiment of theabsorbent article of the invention; and

[0031]FIG. 5 is an explanatory view indicating a contact angle betweenthe surface of a release layer and a water drop thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The invention is described with reference to the accompanyingdrawings. FIG. 1 is a perspective view of one embodiment of awater-decomposable absorbent article of the invention, seen from its topsurface (that shall face the skin of a wearer); FIG. 2 is across-sectional view of the absorbent article of FIG. 1, cut along theline II-II; and FIG. 3 is a perspective view of the absorbent article ofFIG. 1, wrapped and enveloped in a bag of a release sheet. In thesedrawings, a longitudinal direction of the illustrated absorbent articleis designated by Y, and a transverse direction generally perpendicularto the direction Y is designated by X.

[0033] The water-decomposable absorbent article according to theinvention shown in FIG. 1 is for pantiliners or sanitary napkins, andthis is decomposable in water. As shown in FIG. 2, the absorbent article1 comprises an absorbent body 1 a which includes a water-decomposableand liquid-pervious top sheet 10 that shall face the skin of a wearer, awater-decomposable and liquid-impervious back sheet 12, and awater-decomposable absorbent core 11 sandwiched between the top sheet 10and the back sheet 12. In an outer peripheral region 1 e of theabsorbent body 1 a, the top sheet 10 and the back sheet 12 are bonded toeach other with a water-soluble adhesive or via hydrogen bonding formedby thermally embossing them.

[0034] On the outer surface of the back sheet 12, provided iswater-soluble adhesive layers 15 a, 15 b, which is to fit the absorbentbody 1 a of the absorbent article 1 to an external support such as anunderwear while the absorbent article 1 is used. The adhesive layers 15a, 15 b extend in the direction Y of the absorbent article 1 in a lengthof from ½ to {fraction (9/10)} of the longitudinal dimension ofthe-absorbent article 1. To the adhesive layers 15 a,-15 b, directlyfitted is a release sheet 20 that protects the adhesiveness of theadhesive layers 15 a, 15 b until use.

[0035] The release sheet 20 has a three-layered structure, comprising arelease layer 22 (this release layer is also referred to as a lubricantlayer) that faces the adhesive layers 15 a, 15 b; a fibrous layer 24;and a water-soluble polymer layer 23 sandwiched between the releaselayer 22 and the fibrous layer 24.

[0036] The undermost layer, fibrous layer 24 of the release sheet 20 maybe made of water-dispersible fibers of pulp, rayon or the like, or maybe made of non-woven fabric that is prepared by processing a web ofwater-dispersible fibers through water-jet treatment or air-throughtreatment. The water-dispersible fibers for forming the fibrous layer 24may be combined with any other fibers of polylactic acid, polyethylene,polypropylene, polyacryl or polyester.

[0037] Preferably, the fibers constituting the fibrous layer 24 areinsoluble in water. If the fibrous layer 24 contains water-solublefibers, the fibers will absorb water to swell. If so, the water-solublepolymer layer 23 will also swell to be fluid, and will move along withthe swollen water-soluble fibers. More preferably, the fibersconstituting the fibrous layer 24 are hydrophilic so that the sheet 24can readily decompose in water after the release sheet 20 has beendisposed of in water.

[0038] If desired, the fibrous layer 24 may contain a carboxylgroup-having synthetic polymer (e.g., carboxymethyl cellulose) ornatural substance capable of bonding the constituent fibers throughhydrogen bonding therebetween, and may further contain a polysaccharidederivative serving as a binder to enhance the strength of the fibrouslayer 24. For example, an alkali metal salt of carboxymethyl cellulosemay be added to the fibrous layer 24 in an amount of from 0.1 to 15% byweight of the constituent fibers.

[0039] The polymer compound for forming the water-soluble polymer layer23 on the fibrous layer 24 includes, for example, water-solublepolyvinyl alcohol and polyvinyl alcohol derivatives;polyvinylpyrrolidone; polysaccharide derivatives such as methylcellulose and starch; polyacrylic acid, polymethacrylic acid, polymaleicacid and their derivatives; polyether compounds such as polyethyleneglycol. Among those, preferred are polyvinyl alcohol and itsderivatives, as being highly soluble in water and being sealable underheat. Polyvinyl alcohol having a higher degree of saponification is morehighly soluble in water. Therefore, it is desirable that polyvinylalcohol for use herein has a degree of saponification of at least 70%.The hydroxyl groups in polyvinyl alcohol will often form intramolecularor intermolecular hydrogen bonds to lower the solubility in water of thethus hydrogen-bonded polyvinyl alcohol. To solve the problem, therefore,the skeleton chain of polyvinyl alcohol for use herein is preferablygrafted or copolymerized with a functional group of highly hydrophilicoxyethylene or oxypropylene. The thus-grafted or copolymerized polyvinylalcohol is highly soluble even in cold water.

[0040] Preferably, a basis weight (Metsuke) of the water-soluble polymerlayer 23 falls between 5 g/m² and 15 g/m² in order that the polymerconstituting the layer 23 does not gel to form blocks (i.e.,gel-blocking does not occur) in water and the layer 23 is welldecomposable in water. Also preferably, the thickness of the layer 23falls between 5 μm and 20 μm.

[0041] The water-soluble polymer layer 23 is laminated on the fibrouslayer 24 by melt-extruding a water-soluble polymer through a die on thefibrous sheet 24 to form thereon the polymer film having a predeterminedthickness. In the step of lamination through melt extrusion, it is easyto form the water-soluble polymer layer 23 having the desired thicknesson the surface of the fibrous layer 24. In the invention, however, thestep of forming the polymer layer 23 is not limited thereto. The polymerlayer 23 may be formed in any other step, so far as its thickness fallswithin the desired range. For example, a high-viscosity solution of awater-soluble polymer may be applied onto the fibrous layer 24 to formthereon a laminate film of the polymer.

[0042] If, however, the basis weight of the water-soluble polymer layer23 and the thickness thereof are over the desired range as set forthabove, the polymer layer 23 will often gel to form large blocks (i.e.,cause gel-blocking) when the release sheet 20 is disposed of in waterand when the polymer layer 23 is released from the fibrous layer 24therein. If so, it will take a lot of time before the polymer layer 23is dissolved in water. On the other hand, if the basis weight and thethickness of the polymer layer 23 are smaller than the defined range,the influence of the surface roughness of the fibrous layer 24 on theoverlying polymer layer 23 will significantly increase. If so, thesurface of the release layer 22 overlying the polymer layer 23 isdifficult to flatten, and, as a result, the release layer 22 will bedamaged to have surface defects and could not be well resistant towater. Concretely, the equilibrium contact angle between the surface ofthe release layer 22 and a water drop thereon could not be at least 100degrees. This will be described in detail hereinunder.

[0043] In case where the thickness of the water-soluble polymer layer 23falls within the desired range as above, the release sheet 20 is hardlywarped, and, as a result, the release layer 22 is prevented from beingcracked and from having pin holes through it.

[0044] The release layer 22 that overlies the water-soluble polymerlayer 23 may be formed from a fluorine compound or a silicone compound.As being readily formed into the layer 23, preferred is a siliconecompound. Preferably, the basis weight of the release layer 22 is atleast 0.05 g/m² but smaller than 0.3 g/m². With its basis weight beingdefined to fall within the above range, the release layer 22 can satisfythe requirement that the equilibrium contact angle between the surfaceof the layer 22 and a water drop thereon is at least 100 degrees. Therelease layer 22 that satisfies the requirement for the contact angle toa water drop thereon is free from surface defects such as pin holes andhas a flat surface. Another advantage of the release layer 22 having athickness falling within the defined range is that, when thewater-soluble polymer layer 23 has dissolved in water and the layer 22has been thus separated from the fibrous layer 24 therein, the layer 22can be soon dispersed in water. In order to ensure the contact angle ofthe layer 22 defined herein and to ensure easy dispersion thereof inwater, the basis weight of the layer 22 is more preferably from 0.05g/m² to 0.15 g/m².

[0045] The equilibrium contact angle referred to herein is measured witha contact angle meter, “CA-S Micro 2 Model” manufactured by Kyowa KaimenKagaku Co., Ltd. Concretely, as shown in FIG. 5, a water drop 50 is puton the surface of the release layer 22, and, after the two have beenequilibrated (in 2 minutes after dripping), the contact angle θ betweenthe water drop 50 and the surface of the release layer 22 is measured.The instantaneous contact angle referred to in Table 3 in the followingExample is measured in the same manner as set forth above but just after10 seconds.

[0046] If the release layer 22 is damaged to have cracks and pin holesthrough the surface thereof and the underlying water-soluble polymerlayer 23 is exposed out of the release layer 22, the water drop 50 willgradually penetrate into the underlying polymer layer 23, having passedthrough the surface defects of the layer 22, and will be therebyabsorbed by the layer 23. In that case, the contact angle 0 will be soonzero (0) without being equilibrated. If such a damaged layer 22 isfitted to the absorbent article 1 after long-term storage of theabsorbent article 1, the layer 22 could not be smoothly released fromthe absorbent body 1 a as being zipped by its surface defects.

[0047] In case where the silicone compound is used for forming therelease layer 22, it may be applied onto the water-soluble polymer layer23 through gravure coating, flexographic coating or spraying. Amongthese, preferred is gravure coating, as the amount of the compound to beapplied is easy to control and as the compound can form a uniform layer.After the layer 23 has been thus coated or sprayed with it, the siliconecompound is then crosslinked thereon so that it is firmly fixed in theinterface between the-compound layer and the layer 23. For crosslinkingit, the silicone compound layer may be heated at a temperature not lowerthan 100° C. for 1 to 2 minutes, or may be exposed to UV rays (e.g., 350nm, 120 W/cm) for 1 or 2 seconds or so. In case where polyvinyl alcoholis used to form the water-soluble polymer layer in the invention, thecrystallinity thereof will increase under heat and its solubility inwater will be thereby greatly lowered. In that case, therefore, thelatter crosslinking treatment with the UV rays is preferred.

[0048] As shown in FIG. 1, the release sheet 20 formed in the manner asset forth above is rectangular, and has a larger size than the absorbentbody 1 a. The absorbent body 1 a is integrated with the release sheet20, and then inwardly folded at a position 30-30 as shown in FIG. 1,with the thus-folded absorbent body 1 a being inside. Next, this isagain inwardly folded a the position 31-31 in the same manner as above.Being thus folded, the opposite side edges 20 e, 20 e (hatched inFIG. 1) of the release sheet 20 that face to each other are sealed. Inthis case, the release layer 22 is not formed at least in the area ofthe opposite side edges 20 e, 20 e so that the heat-sealable,water-soluble polymer layer 23 of polyvinyl alcohol or the like isexposed outside in that area. With that, the mutually facing releasesheets 20 can be heat-sealed or ultrasonically sealed in the area viathe water-soluble polymer; layer 23 existing therebetween without usingan adhesive.

[0049] An adhesive tape 28 is provided on one edge 20 f of the releasesheet 20 as shown in FIG. 1, and this is fitted to an opposite edge 20 gthereof as shown in FIG. 3. In that manner, the release sheet 20 forms abag 40 that envelops the absorbent body 1 a therein, with the fibrouslayer 24 of the release sheet 20 being outside as shown in FIG. 3. Inthe illustrated embodiment, the release sheet 20 also serves as awrapping sheet for the absorbent article to save the resources.

[0050] Before use, the adhesive tape 28 is removed from the bag 40 as inFIG. 3, and the absorbent article is unfolded to be placed in itsoriginal condition shown in FIG. 1. With that, the release sheet 20 ispeeled off from the absorbent body la so that the adhesive layers 15 a,15 b are exposed outside, and the absorbent body 1 a is fitted to theexternal support such as underwear with the back sheet 12 facing theexternal support. The unnecessary release sheet 20 (wrapping sheet) isdisposed of in flush toilets. In this, the fibrous layer 24 is readilydecomposed in water, and water soon penetrates into the water-solublepolymer layer 23 between the fibrous layer 24 and the release layer 22to dissolve the polymer layer 23, whereby the release layer 2-2 isreadily separated from the polymer layer 23 in water jets and is readilydispersed in water. After used, the absorbent body 1 a can also bedisposed of in flush toilets, as being decomposable in water.

[0051] The top sheet 10 is, for example, made of a water-decomposablenon-woven fabric of spun lace. A plurality of sheets ofwater-decomposable papers may be laminated on a water-decomposablenon-woven fabric to form the top sheet 10. In this case, the non-wovenfabric and the water-decomposable paper sheets may be integrated throughhydrogen bonding or needling. Since the top sheet 10 acts to leadexcretions to the underlying absorbent core 11, it is preferablyperforated to have a plurality of perforations throughout the entirearea thereof as shown in FIG. 1, for example.

[0052] The absorbent core 11 may be made of water-decomposable paper,pulp or non-woven fabric. For example, air-laid pulp or the like may beformed into the absorbent core 11, having a basis weight of from 50 to70 g/m² or so. In case where the absorbent core 11 is made ofwater-decomposable paper, a plurality of sheets of relatively thin,water-decomposable papers may be stacked to form it. This type of theabsorbent core 11 is favorable, as being well decomposable in water. Forexample, about 4 to 8 sheets of water-decomposable papers having a basisweight of from 10 to 20 g/m² are stacked to form the absorbent core 11.As the case may be, sheets of water-decomposable papers coated with awater-swellable resin such as polyvinyl alcohol or the like may bestacked to form the absorbent core 11.

[0053] The back sheet 12 is readily dispersed in water jets in flushtoilets or in water in septic tanks, and it may be formed ofwater-decomposable paper, water-decomposable non-woven fabric or thelike that contains water-dispersible fibers. For example, it may be madeof (1) a water-decomposable paper sheet of pulp fibers where the pulpfibers are bonded to each other via hydrogen bonding therebetween, (2) awater-decomposable paper sheet of pulp fibers and otherwater-dispersible fibers of rayon or the like where the constituentfibers are bonded to each other with a water-soluble binder, (3) awater-decomposable paper sheet of water-dispersible fibers where theconstituent fibers are simply entangled, or (4) a water-decomposablenon-woven fabric of water-dispersible fibers having a relatively shortlength where the constituent fibers are forcedly entangled throughwater-jetting treatment. Preferably, the outer surface of the back sheet12 (this shall face the external support) or the inner surface thereof(this shall face the absorbent core 11) may be coated with awater-soluble resin such as a polyvinyl alcohol, an unsaturatedcarboxylic acid copolymer or the like. Thus coated, the back sheet 12will be impervious to fluid.

[0054]FIG. 4 is a cross-sectional view of another embodiment of theabsorbent article of the invention. In the absorbent article 1A of FIG.4, the release sheet 20A only covers-the adhesive layers 15 a, 15 b. Inthis embodiment, an additional wrapping sheet is required to wrap theabsorbent article 1A. As a wrapping sheet, for example, awater-decomposable fibrous layer, a water-soluble resin or a sheet notdecomposable in water may be used.

EXAMPLES

[0055] The invention is described in more detail with reference to thefollowing Examples, which, however, are not intended to restrict thescope of the invention.

(Example 1)

[0056] A release sheet 20 as shown in FIG. 1 and FIG. 2 was produced todemonstrate one embodiment of the invention. For the fibrous layer 24,used was water-decomposable paper of pulp and rayon. For thewater-soluble polymer layer 23, used was polyvinyl alcohol. This wasmelt-extruded and laminated on the fibrous layer 24. For the releaselayer 22, used was a silicone compound. This was applied onto thewater-soluble polymer layer 23 by gravure-coating, and crosslinked andfixed thereon through exposure to UV rays. The release sheet thusproduced was tested for decomposition in water of the fibrous layer andthe water-soluble polymer layer. The test result is given in Table 1.Comparative samples were also tested in the same manner as herein.

[0057] Decomposition in Water:

[0058] The samples were tested according to the water-decomposabilitytest in JIS P-4501. Precisely, each sample was cut to have a length of10 cm and a width of 10 cm, put into a 300 ml beaker filled with 300 mlof ion-exchanged water, and stirred therein with a stirrer. Therevolution of the stirrer was 600 rpm. While stirred, the sample wasperiodically checked, and the time taken by it until its dispersion inwater was recorded. TABLE 1 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Comp.Ex. 4 Example 1-1 Basis weight of release layer — — — 0.2 0.2 (g/m²)Thickness of water-soluble — 40 20 — 20 polymer layer (μm) Basis weightof fibrous web 40 — 20 40 20 layer (g/m²) Time taken until decomposition12 — 23 not 29 in water of fibrous web layer decomposed (sec) in waterTime taken until decomposition — 138 40 not 180 in water ofwater-soluble decomposed polymer layer (sec) in water

(Example 2)

[0059] Release sheets were produced in the same manner as in Example 1.In these, however, the thickness of the water-soluble layer 23 as wellas the basis weight of the fibrous layer 24 are different. In the testfor decomposition in water of each sample, the area of thenon-decomposed polymer layer 23, if any, was measured. TABLE 2 ExampleExample Example Comp. Ex. Comp. Ex 2-1 2-2 2-3 2-1 2-2 Basis weight ofrelease layer 0.2 0.2 0.2 0.2 0.2 (g/m²) Thickness of water-soluble 1520 20 30 30 polymer layer (μm) Basis weight of fibrous web 20 20 15 2015 layer (g/m²) Time taken until decomposition 25 29 22 38 28 in waterof fibrous web layer (sec) Area of non-decomposed 1 2 2 4 4 polymerlayer (cm²) Time taken until decomposition 120 180 150 720 480 in waterof water-soluble polymer layer (sec)

(Example 3)

[0060] Release sheets were produced in the same manner as in Example 1.In these, however, the basis weight of the release layer 22 differsindividually. A water drop was put onto the release layer 22 of eachsample, and the contact angle, θ, between the surface of the layer 22and the water drop was measured with the contact angle meter, “CA-SMicro 2 Model” manufactured by Kyowa Kaimen Kagaku Co., Ltd., as shownin FIG. 5. The instantaneous contact angle referred to herein is theangle θ measured after 10 seconds; and the equilibrium contact anglealso referred to herein-is the angle θ measured after 2 minutes.

[0061] With its release layer 22 being kept in contact with the adhesivelayers 15 a, 15 b of the absorbent article, the samples were left at 40°C. and 90% RH (relative humidity) for one week and then conditioned at20° C. and 60% RH for 2 hours. The time-dependent peeling strength ofeach sample from the absorbent article was measured according to JISZ0237. In this test, each sample was checked as to whether it was zippedor not when it was released from the absorbent article. For the adhesivelayers 15 a, 15 b, used was an acrylic resin adhesive. TABLE 3 Comp.Example 3-1 Example 3-2 Example 3-3 Example 3-4 Example Basis weight ofrelease layer 0.3 0.2 0.15 0.1 0.05 (g/m²) Thickness of water-soluble 1515 15 15 15 polymer layer (μm) Basis weight of fibrous web 20 20 20 2020 layer (g/m²) Time taken until decomposition 42 25 21 17 — in water offibrous web layer (sec) Area of non-decomposed 6 1 1 — polymer layer(cm²) Time taken until decompositon 240 120 40 17 — in water ofwater-soluble polymer layer (sec) Initial peeling strength 55.1 54.9056.3 180.4 332.4 (mN/inch) Zipping no no no no yes, a little Peelingstrength after one week 65.6 87.2 112.8 216.7 560.9 (mN/inch) Zipping(after one week) no no no no yes Instantaneous contact angle 107.5 107.7102.9 (degrees) Equilibrium contact angle 107 106.5 0 (degrees)

[0062] From the test results as above, it is understood that the releasesheet of the invention is, even after stored long, not zipped and canwell protect the adhesive layer of absorbent articles for a long periodof time. In Example 3, the release sheet was able to be heat-sealed.Therefore, this type of the release sheet can be used for individuallywrapping the absorbent article, and can be heat-sealed in a simplifiedmanner to provide individually wrapped absorbent articles.

[0063] As described in detail hereinabove, the release sheet forwater-decomposable absorbent articles of the invention is decomposablein water. Therefore, it can be disposed of in toilets without requiringtreating as wastes, and is therefore convenient to users. Having theadvantage of good decomposability in water, another advantage of therelease sheet is that it is resistant to water and is therefore hardlydamaged to have surface defects such as cracks and pin holes. Cracks andpin holes, even if formed in the release sheet, do not enlarge. Evenafter stored for a long period of time, the release sheet well has itsreleasability from the adhesive layer of absorbent articles.

[0064] The absorbent article of the invention may be individuallywrapped with the release sheet fitted thereto. Thus wrapped, theabsorbent article is convenient to users, since the unnecessary part foruse may be reduced and the wrapping sheet can be disposed of in toilets.Specifically, in case where the water-soluble polymer layer of therelease sheet is made from a thermoplastic resin, this type of therelease sheet can be heat-sealed for individual wrapping, therebyfacilitating production of the absorbent articles.

[0065] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

[0066] Here, ‘comprises/comprising’ when used in this specification istaken to specify the presence of stated features, integers, steps orcomponents but does not preclude the presence or addition of one or moreother features, integers, steps, components or groups thereof.

What is claimed is:
 1. A water-decomposable absorbent articlecomprising: an absorbent body that includes a water-decomposable topsheet, a water-decomposable back sheet, a water-decomposable adsorbentcore sandwiched between the top sheet and the back sheet, and anadhesive layer provided on the back sheet to fit the absorbent articleto an external support; and a water-decomposable release sheet forprotecting the adhesive layer before use with covering the adhesivelayer, wherein; the release sheet includes a release layer that facesthe adhesive layer, a water-decomposable fibrous layer, and awater-soluble polymer layer sandwiched between the release layer and thefibrous layer, a basis weight of the release layer is at least 0.05 g/m²but smaller than 0.3 g/m², and an equilibrium contact angle between asurface of the release layer and a water drop thereon is at least 100degrees.
 2. The water-decomposable absorbent article as set forth inclaim 1, wherein the release layer is formed from a silicone compound,and a basis weight thereof is at most 0.15 g/m².
 3. Thewater-decomposable absorbent article as set forth in claim 1, wherein athickness of the water-soluble polymer layer falls between 5 μm and 20μm.
 4. The water-decomposable absorbent article as set forth in claim 1,wherein the water-soluble polymer layer is formed from a polyvinylalcohol, and the polyvinyl alcohol has a degree of saponification of atleast 70%.
 5. The water-decomposable absorbent article as set forth inclaim 4, wherein the release sheet is to be a wrapping sheet forentirely wrapping and enveloping the absorbent article, and has oppositeside edges which are heat-sealed via the polyvinyl alcohol.
 6. A methodfor producing a water-decomposable absorbent article that includes awater-decomposable top sheet, a water-decomposable back sheet, awater-decomposable absorbent core sandwiched between the top sheet andthe back sheet, and an adhesive layer provided on the back sheet to fitthe absorbent body to an external support, which comprises; a step ofmelt-extruding and laminating a water-soluble polymer layer having athickness of from 5 μm to 20 μm on a surface of a water-decomposablefibrous sheet made of hydrophilic and water-insoluble fibers, a step ofapplying a release material onto a surface of the water-soluble polymerlayer to form thereon a release layer having a weight of at least 0.05g/m² but smaller than 0.3 g/m², a step of crosslinking the releasematerial such that an equilibrium contact angle between the surface ofthe release layer and a water drop thereon is at least 100 degrees,thereby forming a release sheet, and a step of fitting the release sheetto the adhesive layer of the absorbent body via the release layer.